A selector mechanism generally comprise a selector connected to position sensors so as to select a selector mode, for instance Park (P), Reverse (R), Neutral (N) or Drive (D). The position sensors are connected to a transmission control unit supplying control signals to an actuator such as a solenoid and a motor in order to drive a transmission in accordance with output signals from the position sensors. Such vehicle transmissions when operated by shift-by-wire commonly include a wide variety of sensors, switches, and other controls. In the event that any of these components fails, the transmission may fail to function. If such a failure is detected, the transmission control unit switches to a failure or ‘limp home’ mode which is designed to ensure that, as far as possible, the vehicle will be drivable and safe. Generally in this mode the transmission control will attempt to maintain the current gear and the failure is indicated to the driver by means of a warning light or text message.
Despite the various warning alerting the driver that the transmission is in limp-home mode, it is still possible for the selector to be moved to a position to select an inappropriate selector mode, possibly causing immediate and unexpected motion of the vehicle.
According to the particular type of selector mechanism, several alternatives have been proposed to overcome this problem. One alternative is disclosed on the Aston Martin™ DB9™ where the selector takes the form of individual switches on the dashboard, each for selecting an operating mode for the transmission. In this alternative, the transmission control unit stops the engine if the driver selects an operating mode which is not available. In the other forms of selector, i.e. a lever or a rotary knob, the driver must move the selector through a Neutral position to get a Drive position or a Reverse position. In such cases, if the transmission is in limp-home mode and the driver moves the selector towards a mode which is unavailable the transmission control unit will shift the transmission to Neutral. Both of these known types of alternative have certain inherent disadvantages for driver convenience and safety.
The present invention has as an object the provision of an improved selector mechanism for a vehicle transmission having an improved interaction with the driver when the transmission is in a limp-home mode.